The development of communication systems over time has been central to the progress of human civilization. The evolution of communications has largely been a story of technological evolutions that have served to increase the bandwidth, speed and distance of communications. The era of electronic communications began with Morse's development of the telegraph, a technology that provided instantaneous communication, but at a rate of only a few bits per second. Bell's telephone, developed in 1870's, was the first major extension of the telegraph and provided a 4 kHz bandwidth. Trends in communications continued to evolve to higher frequency carrier waves, which have proportionally higher bandwidths for carrying information. Early radio operated in the 1 MHz range and provided a bandwidth of 15 kHz. Analog television requires a bandwidth of about 6 MHz and was achievable through carrier frequencies distributed from 54–806 MHz. Advances in communications continued through the advent of semiconductor electronics and the development of the transistor in the 1950's. Silicon-based electronic devices offer bandwidths in the GHz range.
It has long been realized that communications based on an optical source would offer far greater bandwidth than electronic systems. Optical frequencies are in the range of a few hundred THz, frequencies that greatly exceed the switching and modulation speeds of electronic systems. Major steps toward the realization of optical communications systems include the development of waveguides and optical fibers, including the identification and processing of materials having suitably low absorption and scattering losses, and the advent of the laser. Achievement of THz range communication frequencies has, however, proven to be more challenging than expected as the initial and most of today's optical communication systems are optoelectronic in nature and rely on electronic components (e.g. switches, modulators, controllers, amplifiers, connectors, filters) to control light. These components introduce an electronic bottleneck that limits the ability of the communications industry to realize the full potential of optical carrier frequencies. Although the capabilities of electronic components have greatly improved over the past several decades, it is widely believed that electronic devices are quickly approaching their ultimate speeds. Further improvements in bandwidth require new strategies, unconstrained by the limitations of electronics, for controlling light. Ideally, an all-optical system is desired.
In order to achieve an all-optical communications network, it is necessary to develop optical analogs or successors of the existing electronic devices used in today's optoelectronic systems and to package those components in an efficient, compact system. Much of today's effort is directed at developing optical connectors, switches, modulators etc. at the device level and in integrating these devices in a manner analogous to the successful designs used in integrated electronic circuits. From a processing point of view, planar structures are desired and as a result, the planar waveguide is the primary conduit for directing light in an integrated optical system. The planar waveguide becomes the optical analog of the wire interconnect of an integrated electronic circuit. Transmission of light over large distances, however, is accomplished by directing light through single-mode optical fibers since it is now possible to economically fabricate low loss optical fibers having kilometer scale lengths.
In most designs of an all-optical communication network, light is transferred between integrated optical components using optical fibers. The integrated optical components are active elements that generate, process and detect optical signals and the optical fibers are passive elements that serve to route light between the integrated optical components in a network. The successful implementation of an all-optical communication network thus requires an efficient coupling of optical fibers to integrated optical components, especially planar components. Since the input optical signals received by an integrated optical component are initially introduced into a waveguide, a central issue in the coupling of fibers to planar integrated optical components is the efficient transfer of light from a fiber into a planar waveguide.
The fiber-waveguide junction is a key source of loss in an all-optical system. The origin of the losses arises from differences in the propagation characteristics of light in different optical components due to differences in the optical medium and confinement. Optical fibers, for example, transport optical modes having a large diameter power profile due to the weak confinement provided by the small refractive index difference between the core and cladding. Integrated waveguides, in contrast, typically use a guiding material (e.g. silicon) having a higher index of refraction than the material used in a fiber (e.g. silica) and/or stronger confinement, with the result that the guided mode diameter is much smaller in an integrated waveguide than in a fiber. Typically, the mode diameters of a single-mode silica fiber and a silicon waveguide are 10 μm and 0.5 μm, respectively. Efficient coupling of a fiber to a waveguide thus requires a mechanism for transforming a mode having a characteristic fiber diameter into a mode having a characteristic waveguide diameter and vice versa. The potential for losses in the transformation is high, especially in the fiber to waveguide direction since a large diameter beam needs to be converted to a small diameter beam without substantial loss of power. Simple end-coupling (back-to-back placement) of a fiber to an integrated waveguide leads to substantial losses due to the large mismatch in cross-sectional area between the fiber and the waveguide. Much of the signal exiting the fiber is unable to enter the much smaller waveguide due to a mismatch in the physical dimensions or cross-sectional spatial overlap at the point of transfer between the fiber and waveguide. Some of the light exiting a large diameter fiber necessarily bypasses a waveguide having a smaller cross-section in at least one direction and this light necessarily represents a loss in optical signal.
One solution in the prior art for adjusting mode diameter in the transfer of an optical signal from a fiber to a waveguide is tapering. The purpose of tapering is to adjust the physical dimensions of the fiber or waveguide so that a better match in mode diameter is achieved. In fiber-tapering, the goal is to decrease the mode diameter by narrowing the output end through a reduction in physical dimensions to provide a better match for the acceptance aperture of an integrated waveguide. In this way, better cross-sectional overlap of the beam exiting the fiber and beam optimally guided by the waveguide is achieved. Lensing fibers have similarly been used for this purpose. Although fiber-tapering and lensing provide improvements, the achievable mode diameters are limited by the beam waist limit in free space and this limit is still larger than the aperture of a typical silicon waveguide. Alignment requirements are also stringent.
An alternative solution in the prior art is waveguide tapering. In waveguide tapering, the goal is to increase the mode diameter at the acceptance or receiving end of the waveguide so that a better match with the fiber is achieved. Waveguide tapering can be accomplished through up-tapering or down-tapering. In an up-tapering configuration, the ends of a planar waveguide are ramped or flared outward and upward to match the physical dimensions of the fiber core to provide increased cross-sectional overlap. The flared ends receive the large diameter beam from the fiber and focus it down gradually to the desired sub-micron dimensions as the flared ends converge and merge with the planar waveguide. The flared ends similarly work in reverse and convert a small diameter waveguide mode into a large diameter mode that can be transmitted to a fiber.
The practical difficulty with the up-tapering configuration is processing. Up-tapering requires tall out-of-plane structures that are complicated to fabricate in a planar processing technology. Another problem is that long tapers are needed for mode conservation and to minimize losses. Long tapers provide a gradual transformation of mode diameter and minimize the tendency of the transferred mode from distributing its power among the many allowed modes of the integrated waveguide. Distribution of the fiber mode into two or more of the allowed or radiation modes of the waveguide leads to scattering losses. These losses can be minimized only through the formation of long, extended tapers (on the mm scale). Such tapers consume large amounts of the available area of an integrated optical device.
Waveguide tapering can also be achieved in a down-tapering configuration in which the receiving end of the waveguide is decreased to a tip having a size in the nanometer regime that is sufficiently small to permit a delocalization of the mode field. The delocalization leads to an expansion of the waveguide mode beyond waveguide core and into the cladding to provide better spatial overlap with the fiber core. The drawbacks of down-tapering are the processing complexities inherent in forming the required nanoscale tapered tip and alignment difficulties.
Any mismatch in the refractive index between the fiber core and integrated waveguide leads to reflection losses that further reduce the efficiency of power transfer.
The deficiencies in the prior art methods for coupling optical fibers to integrated waveguides demonstrates a need for new coupling devices that provide a more efficient coupling of modes between optical fibers and waveguides. Ideally, the coupling devices should provide for the efficient transfer of power back and forth between a fiber and a waveguide while preserving the mode symmetry and avoiding scattering and reflection losses. In many applications, it is further desired to produce a mode of a desired polarization in the waveguide (e.g. TE or TM) from the randomly polarized beam emerging from a fiber.